[flax] Add messages to some assertions in pool to help with debugging.

[copybara-service]

[flax] Add messages to some assertions in pool to help with debugging.

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[codecov-commenter]

Codecov Report

Merging #1321 (ecd0c4a) into master (d5f12e9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1321   +/-   ##
=======================================
  Coverage   82.33%   82.33%           
=======================================
  Files          65       65           
  Lines        5316     5316           
=======================================
  Hits         4377     4377           
  Misses        939      939           

Impacted Files	Coverage Δ
flax/linen/pooling.py	86.66% <100.00%> (ø)

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